I do not believe that any one area is more important than another. There is a symbiotic relationship between these steps because the build on each other and take up where the last one left off. If I have to choose a step as being more crucial than another it would be step three. It is imperative that you trouble shoot your ideas for flaws that exist and work out as many of the kinks as possible be for practical application can begin. Otherwise the headache that is created is usually crippling to the entire process regardless of how good of an idea it was to start with.
Bateman, T.S., & Snell, S.A. (2011). Management: Leading & collaborating (9th ed.). Management: Leading & collaborating in a competitive world , New York: McGraw-Hill